Break the Repeating-Key XOR Cipher

Repeating-Key XOR: A Classic Weakness

XOR encryption is beautifully simple: every byte of plaintext is XORed with a byte from a repeating key. If the key is short and the plaintext is known to follow a predictable pattern (like printable ASCII), the cipher crumbles quickly.

Why It Breaks

When the same key byte encrypts many plaintext bytes, statistical patterns survive. For a key length of n, bytes at positions 0, n, 2n, … were all encrypted with the same key byte. If the plaintext is ASCII, the most common byte in each group is likely 0x20 (space) or the character e — XORing your guess against the most-frequent ciphertext byte reveals the key byte. Repeat for each position and you recover the full key.

How to Decode

Your intercepted blob is delivered as a hex string. The workflow is:

1. Hex-decode the blob — in Python: bytes.fromhex(blob) or CyberChef's From Hex operation.
2. Identify key length — try lengths 1–8; for each, split ciphertext into columns and score each column's byte frequencies against expected English/ASCII.
3. XOR each column with the best-guess key byte to recover the key, then XOR the full ciphertext with the repeating key.
4. In Python: ''.join(chr(b ^ ord(k)) for b, k in zip(ct, itertools.cycle(key))) — the flag will appear in the plaintext.

Tools

• Python 3 (recommended): itertools.cycle, bytes.fromhex, list comprehensions.
• CyberChef: chain From Hex → XOR with your discovered key set to UTF-8 / Cycling.
• xortool (pip install xortool): automates key-length detection and key recovery in one step.